Why are Tasks not able to be ranked on a custom Kanban board or custom list?
This is working as designed. Tasks can only be ranked against other tasks within the context of the user story or defect to which they are assigned.
For example US123 has tasks TA1 and TA2 associated to it; while US456 has TA3 and TA4 associated. TA1 cannot be ranked against TA3 or TA4. TA1 can be ranked relative to TA2 but only in the context of US123. Likewise TA3 and TA4 can be ranked only to themselves relative to US456.
The Task Board app does allow for tasks to be ranked easily, relative to tasks that are associated to the same artifact.